In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) is heretofore used. Specifically, the PS plate is exposed through a mask, for example, a lith film, and then subjected to development processing, for example, with an alkaline developer to remove the unnecessary image-recording layer corresponding to the non-image area by dissolving while leaving the image-recording layer corresponding to the image area, thereby obtaining the lithographic printing plate.
Due to the recent progress in the technical field, nowadays the lithographic printing plate can be obtained by a CTP (computer-to-plate) technology. Specifically, a lithographic printing plate precursor is directly subjected to scanning exposure using laser or laser diode without using a lith film and developed to obtain a lithographic printing plate.
With the progress described above, the issue on the lithographic printing plate precursor has transferred to improvements, for example, in image-forming property corresponding to the CTP technology, printing property or physical property. Also, with the increasing concern about global environment, as another issue on the lithographic printing plate precursor, an environmental problem on waste liquid discharged accompanying the wet treatment, for example, development processing comes to the front.
In response to the environmental problem, simplification of development or plate making or non-processing has been pursued. As one method of simple plate making, a method referred to as an “on-press development” is practiced. Specifically, according to the method after exposure of a lithographic printing plate precursor, the lithographic printing plate precursor is mounted as it is on a printing machine without conducting conventional development and removal of the unnecessary area of image-recording layer is performed at an early stage of printing process.
In the simplification of plate making operation as described above, a system using a lithographic printing plate precursor capable of being handled in a bright room or under a yellow lamp and a light source is preferred from the standpoint of workability. Thus, as the light source, a semiconductor laser emitting an infrared ray having a wavelength of 760 to 1,200 or a solid laser, for example, YAG laser, is used. An UV laser is also used.
As the lithographic printing plate precursor capable of undergoing on-press development, for instance, a lithographic printing plate precursor having provided on a hydrophilic support, an image-recording layer (heat-sensitive layer) containing microcapsules having a polymerizable compound encapsulated therein is described in Patent Documents 1 and 2. A lithographic printing plate precursor having provided on a support, an image-recording layer (photosensitive layer) containing an infrared absorbing dye, a radical polymerization initiator and a polymerizable compound is described in Patent Document 3. A lithographic printing plate precursor capable of undergoing on-press development having provided on a support, an image-recording layer containing a polymerizable compound and a graft polymer having a polyethylene oxide chain in its side chain or a block polymer having a polyethylene oxide block is described in Patent Document 4.
In general, an operation (plate inspection) for inspection and discrimination of image formed on a printing plate is carried out in order to examine whether the image is recorded on the printing plate as intended, in advance of mounting the printing plate on a printing machine. In a conventional lithographic printing plate precursor subjected to a development processing step, since a color image is ordinarily obtained due to the development processing by means of coloration of the image-recording layer it is easily performed to confirm the image formed before the mounting the printing plate on a printing machine.
With respect to the lithographic printing plate precursor of the on-press development type or non-processing (non-development) type without accompanying the development processing, the image is not recognized on the printing plate in the step of mounting it on a printing machine, and thus the plate inspection cannot be performed. In particular, it is vital in the printing operation to determine whether a registry guide (register mark) which acts as a landmark for the register in multicolor printing is recorded. Therefore, in the lithographic printing plate precursor of the on-press development type or non-processing (non-development) type, a means for confirming the image, that is, color formation or decoloration in the exposed area to form a so-called print-out image is required at the stage of exposure. Further, from the standpoint of improvement in the workability, it is required that the exposed area color-formed or decolored does not change after the lapse of time and the color-formed or decolored state is maintained.
A lithographic printing plate precursor is proposed wherein a compound capable of generating an acid, base or radical by means of light or heat and a compound capable of undergoing color change upon interaction with the acid, base or radical generated are used as a print-out agent (for example, see Patent Document 5). Also, it is proposed to utilize color change of thermally decomposable compound as the print-out agent of a direct-drawing type lithographic printing plate precursor having a heat-sensitive layer (for example, see Patent Document 6). Further, it is proposed to use a thermally decomposable dye having a thermally decomposable temperature of 250° C. or below as the print-out agent (for example, see Patent Document 7).
According to these proposals, although the color formation or decoloration occurs in the exposed area and the plate inspection property increases to some extent, it is still insufficient.
It is described in Patent Document 8 that a print-out image having a good visibility and a level capable of performing plate inspection is obtained by a system containing an infrared absorbing dye of cyanine dye having a 5-membered ring in its methine chain and a radical generator. However, the technique is still insufficient in view of maintaining the good visibility after the lapse of time.
It is described in Patent Document 9 that good visibility is obtained by a compound having a property of decomposing upon heat or light stimulus. It is described that the compound causes change in the π conjugated system at the time of decomposition to form a colored body. However, the visibility according to the technique is still insufficient in comparison with the conventional lithographic printing plate precursor subjected to a development processing step, and a higher color-forming property is required.